In many wireless communication systems, it is desirable to capture received signal information in a digital format so that digital signal processing (“DSP”) techniques can be applied to that signal. The digital format makes it possible to manipulate the signal and extract useful information in ways that cannot be achieved with analog signal processing devices and elements. Because RF signals are received in an analog domain, however, wireless communication systems often use Analog-to-Digital Converters (“ADC”) to convert the received RF signal to digital information.
The performance of the ADC is characterized by both resolution (the number of bits used to quantize the RF signal) and sample rate (the speed at which the ADC makes measurements of the signal). Fundamental relationships between the sampling rate and the bandwidth of the signal being measured must be maintained in order for the data from the ADC to be correct and useful.
Unfortunately, ADC performance is insufficient for direct sampling techniques (e.g. Nyquist or Bandpass) at higher RF frequencies, as performance of high speed ADCs are limited to sampling rates below ˜3 Giga-Samples Per Second, and resolutions of less than 8 bits (with resolution improving at the expense of sample rate and vice versa). Accordingly, frequency conversion of an RF signal may require a down converter, generally composed of one or more local oscillators and mixers (such as in a superheterodyne receiver) to perform frequency conversion. In complex signal environments (e.g. a range of signal modulation formats having bandwidths and power levels in adjacent and/or even overlapping spectrum locations), non-linear effects in the mixers create intermodulation products that limit system performance. In addition, the quality of the local oscillators also affect performance, with high quality components being expensive and often subject to environmental effects that can degrade performance. Also, if bandpass sampling is applied in a complex signal environment, aliasing effects from multiple signals can fall within the same sampled spectrum and degrade the sampled data, preventing the signal of interest from being correctly captured.